Zinc Single Atom Confinement Effects on Catalysis in 1T-Phase Molybdenum Disulfide

Active sites are atomic sites within catalysts that drive reactions and are essential for catalysis. Spatially confining guest metals within active site microenvironments has been predicted to improve catalytic activity by altering the electronic states of active sites. Using the hydrogen evolution reaction (HER) as a model reaction, we show that intercalating zinc single atoms between layers of 1T-MoS2 (Zn SAs/1T-MoS2) enhances HER performance by decreasing the overpotential, charge transfer resistance, and kinetic barrier. The confined Zn atoms tetrahedrally coordinate to basal sulfur (S) atoms and expand the interlayer spacing of 1T-MoS2 by ∼3.4%. Under confinement, the Zn SAs donate electrons to coordinated S atoms, which lowers the free energy barrier of H* adsorption–desorption and enhances HER kinetics. In this work, which is applicable to all types of catalytic reactions and layered materials, HER performance is enhanced by controlling the coordination geometry and electronic states of transition metals confined within active-site microenvironments.


INTRODUCTION
Hydrogen (H 2 ) is recognized as an essential green energy carrier that is widely used as a chemical feedstock in petroleum refinement, fertilizer production, and as a fuel source for electricity/heat generation. 1 As of 2021, 95% of H 2 produced in the United States is generated by steam−methane reformation. 2 In addition to being energy-intensive, this centralized process increases CO 2 emissions and requires large power plants that are expensive to build. Consequently, this method of H 2 generation is not accessible to countries that lack these resources. 3 Luckily, alternative pathways to H 2 generation exist. 4 Using electrochemical methods to drive the H 2 evolution reaction (HER) provides a sustainable, decentralized alternative by requiring only water to yield H 2 with high purity. 5,6 As such, it circumvents the challenges of steam−methane reformation and enables countries with limited resources to become more selfreliant. Before electrochemical H 2 generation can be implemented in society, critical obstacles still need to be overcome. Primarily, electrocatalysts that are cost-effective, highly efficient, and durable must be developed to replace the precious metal electrocatalysts currently employed in commercialized electrolyzers. 7−9 Since the performance of electrocatalysts depends on the nature of their active sites, methods that maximize active site performance in the next generation of electrocatalysts need to be established.
Theoretical studies have revealed that spatially confining microenvironments (the local coordination environment and electronic states of active sites) within catalysts can enhance the catalyst's activity by modulating the frontier orbital energies and adsorption−desorption energies of active sites. 10,11 Electronic properties of confined active sites are directly influenced by their coordination environment, which in turn alters the adsorption energetics of reaction intermediates and catalytic activity/selectivity. 12 Therefore, the relationship among active sites, local microenvironments, and confined species dictates catalytic performance. However, designing a prototype system that enables understanding of catalytic confinement effects at the atomic level remains challenging.
Single-atom catalysts (SACs), in which single atoms (SAs) are stabilized within supporting substrates by either adsorbing to the substrate's basal plane or substituting atoms within the substrate's lattice, offer an ideal prototype for the investigation of confinement catalysis. Along with demonstrating a superior catalytic performance compared to nanoparticles and nanoclusters in traditional metal catalysts, SACs exhibit flexibility with respect to crystallinity, coordination number, and electronic structures. 13 Likewise, when employing noble metal SAs, SACs require significantly smaller quantities of noble metals to achieve competitive catalytic performance to produce solar fuels and industrial chemicals.
To understand the confinement effects between catalysts and their microenvironments, identifying a paradigmatic support material is crucial. Commonly employed scaffolds that provide spatial confinement include channels in carbon nanotubes (CNTs) and porous sites in zeolites and metal organic frameworks (MOFs). 12,14 In these cases, methods such as doping the catalyst with nonmetals, forming bimetallic active sites, and synthetically inducing an anisotropic catalyst surface have all been shown to enhance the catalytic activity. 15−17 However, both zero-dimensional (0D) nanocavities in zeolites/MOFs and one-dimensional (1D) nanocavities in CNTs suffer from major disadvantages such as complex structural and chemical composition. 12,14 These complexities create an uneven environment surrounding the active sites and make understanding confinement effects very difficult at the microscopic level. 12,14 Compared to their three-dimensional (3D) counterparts, two-dimensional (2D) materials exhibit well-defined layered structures, a variety of polymorphs, and tunable geometric and electronic properties. Computational studies have predicted the interactions between active sites and guest species confined within their local microenvironments to heavily influence catalytic activity. 18−20 Yet, experimental evidence of 2D materials other than carbon-based 2D materials is limited. 21 For these reasons, exploring other 2D materials for confinement studies would provide an ideal platform to understand how confining guest species near active sites located within the substrate's interlayer spacing influences catalytic performance.
HER is an ideal model reaction for confinement studies due to the fast kinetics of proton (H + ) diffusion that occurs between layers of 2D materials. 14 Customarily, the HER activity of HER catalysts is evaluated using hydrogen adsorption free energies (ΔG H* ). Both nonmetal and transition metal HER catalysts follow the same trend, where maximum HER activity is achieved at around ΔG H* = 0 eV. 7,22 In the continuous search for Earth-abundant catalysts, MoS 2 serves as a role model in HER catalysis. 7−9 For decades, its activity has been considered limited due to the extremely high energy of proton adsorption on the basal plane of semiconducting 2H-MoS 2 (ΔG H* = 1.92 eV). This changed when theoretical calculations revealed the extremely thermoneutral nature of the edge sites in 2H-MoS 2 (ΔG H* = 0.08 eV). 23 Volcano plots published in the literature that are used to access HER activities of metal nanoparticles and other HER catalysts have demonstrated that, compared to other commonly employed nonprecious HER catalysts, molybdenum dichalcogenides maintain a ΔG H* nearest to zero. 7,9,24,25 Therefore, MoS 2 is considered to be an ideal candidate because of the fast HER kinetics that it sustains. The best performing HER catalysts are precious metals which are rare and expensive, such as platinum (Pt). In contrast, MoS 2 is composed of Earth-abundant elements and thus is much less expensive and more feasible at scale. For these reasons, MoS 2 is considered to be more advantageous than other high-performance HER catalysts.
Various strategies have been employed to expose more active edge sites in 2H-MoS 2 . 26,27 For instance, Wang et al. employed a mild H 2 O 2 chemical etching strategy to investigate the impact of both the concentration and distribution of S vacancies in MoS 2 on HER activity. 28 The results suggest that the homogeneous distribution of single S-vacancies throughout the MoS 2 nanosheet surface achieves optimal HER performance, as demonstrated by the 48 mV/dec Tafel slope and 131 mV overpotential reported. Subsequently, the direction of MoS 2 -based HER research shifted with the discovery of the metallic 1T-phase of MoS 2 , due to the higher density of active sites available along the basal plane of 1T-MoS 2 . 29,30 Compared to the trigonal prismatic 2H-phase of MoS 2 , 1T-MoS 2 layers feature well-defined octahedral symmetry, which increases the exposure of surface active sites for enhanced catalytic performance. 31 Furthermore, metallic 1T-MoS 2 exhibits exceptional charge transport properties compared to its 2H-phase semiconducting analogue, thus enabling further exposure of active sites, at which surface reactions take place. 32,33 While 2H-MoS 2 is known to be the more thermodynamically stable phase in nature, intercalation of SAs between layers of MoS 2 enables MoS 2 to remain stable in the 1T-phase. 34 In addition, the distinct, local atomic environment and uniform chemical nature of SAs offer advantages of distinguished activity, selectivity, and stability studies for the HER.
Thus far, only a few catalytic confinement studies have utilized MoS 2 as the host material. For example, Chen et al. intercalated Pt nanoparticles within the van der Waals gaps of bulk MoS 2 and discovered that confinement not only suppressed the aggregation of Pt nanoparticles but also facilitated the transfer of H 3 O + during HER. 35 Likewise, Luo et al. inserted Co(OH) 2 nanoparticles between layers of bulk MoS 2 to improve the HER performance of MoS 2 under alkaline media. 36 Unfortunately, previous confinement studies suffer from either utilization of precious metals or difficulty with controlling the size and structure of intercalated species, resulting in an unsystematic study of confinement effects.
Other works have investigated the catalytic effects of modifying 1T-MoS 2 with first-row transition metals. Huang et al. discovered that hydrothermally synthesizing 1T-MoS 2 with Fe, Co, and Ni enhances HER activity in alkaline media by doping the guest metals into the 1T-MoS 2 lattice in a 1:6 X:Mo ratio (X = Fe, Co, or Ni). 37 Li et al. reported the ability to enhance the HER performance of 1T-MoS 2 by either substituting lattice sites with copper (Cu) SAs or adsorbing Cu SAs along the 1T-MoS 2 basal plane. 38 Each of these methods of stabilizing Cu SAs was achieved by employing syringe injection and hydrothermal synthetic methods, respectively. Meanwhile, in a volcano plot reported by Deng et al., Zn demonstrated a ΔG H* value near 0 eV, indicating that it is one of the few nonprecious metals that may be able to modify the HER activity of MoS 2 . 39 Therefore, Zn SAs were selected as the guest intercalants in this study to investigate the effects of nonprecious metal confinement effects on HER catalysis.
In this work, Zn SAs were intercalated within the interlayer spacing of 1T-MoS 2 (Zn SAs/1T-MoS 2 ) via syringe injection into hydrothermally synthesized 1T-MoS 2 (see the Methods). The results reported herein show that the confined SAs maintain a Zn 2+ oxidation state and expand the interlayer spacing of 1T-MoS 2 by ∼3.4% (0.022 nm). Changes in catalytic performance during HER were monitored electrochemically under acidic conditions, where a decrease in overpotential (1T-MoS 2 = 265 mV; Zn SAs/1T-MoS 2 = 177 mV) and charge transport limitations (1T-MoS 2 = 106.4 mV/ dec; Zn SAs/1T-MoS 2 = 84.9 mV/dec) were observed when Zn SAs were intercalated between 1T-MoS 2 layers. The experimental findings were further confirmed computationally using proton adsorption energies predicted from firstprinciples density functional theory (DFT) and partial density of states (PDOS) plots. In addition to facilitating HER kinetics, the spatial confinement of Zn SAs was predicted to enhance interactions between protons and the microenvironments of nearby active sites within which they are confined within.

RESULTS AND DISCUSSION
Adsorption of Zinc Single Atoms along the Basal Plane of 1T-MoS 2 . SAs may be stabilized on appropriate substrates by either substituting lattice vacancies or adsorbing to the substrate's basal plane. In this work, high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) was used to identify the positions occupied by Zn SAs in the 1T-MoS 2 lattice (Figures 1, S1). In the HAADF-STEM image shown in Figure 1a, the intensity of each lattice site is correlated to the atomic number of elements occupying the lattice site. 40 If the Zn SAs substitute Mo lattice sites, the lattice vacancies occupied by Zn SAs will appear darker since Zn (Z Zn = 30) has a lower atomic number than Mo (Z Mo = 42). Instead, brighter spots are observed, indicating that the Zn SAs adsorb to the basal plane and occupy sites above the Mo atoms. Additional evidence that the Zn SAs adsorb to the basal plane of 1T-MoS 2 is shown by the HAADF-STEM images provided in Figure S1  HAADF-STEM images collected under continuous electron beam irradiation revealed the ability to trigger the migration of Zn SAs across the 1T-MoS 2 basal plane ( Figure S2). When the electron beam was focused on the same region for 109 s, the Zn atom located in L4 (t = 0 s, designated with a red arrow in Figure S2a) jumps to L5 (t = 109 s, Figure S2b). After 203 s, only one of the four Zn atoms (designated by the yellow arrow) present remains in the same position, while the other three Zn atoms migrate out of the scope area ( Figure S2c). These observations further confirm that rather than substituting into the lattice, the Zn SAs adsorb to the basal plane of 1T-MoS 2 . It is worth mentioning that the dynamic movement of SAs under electron beam conditions is well-known. 41  persist under catalytic conditions. However, a spectroscopic or microscopic technique needs to be developed that can confirm the dynamics of SAs while supplying the minimum amount of external energy required to trigger their movements.
Expansion of 1T-MoS 2 Lattice Fringe Spacing by Intercalated Zinc Single Atoms. By convention, intercalation reactions are characterized by the expansion of a layered substrate's crystal lattice along the c axis. 44 Expansion of 1T-MoS 2 's interlayer spacing was evidenced by visible peak shifts in the X-ray diffraction (XRD) patterns of Zn SAs/1T-MoS 2 compared to that of 1T-MoS 2 (Figures 2a, 2b). Peak broadening observed in the diffraction pattern of bulk Zn SAs/1T-MoS 2 compared to that of 1T-MoS 2 arises from the overlap of hexagonal 1T-MoS 2 (P6 3 /mmc; PDF no. 75-1539) and monoclinic Mo 2 S 3 (P2 1 /m; PDF no. 72-0821). Mo 2 S 3 consists of molecular Mo−S chains intercalated between 1T′-MoS 2 layers ( Figure S3). 45 Interestingly, the peak overlap observed in 1T-MoS 2 splits into two separate peaks in Zn SAs/ 1T-MoS 2 : the (001) plane bisecting the interlayer spacing of 1T-MoS 2 and the (101̅ ) plane bisecting the Mo−S chains that intercalate the 1T'-MoS 2 layers in Mo 2 S 3 . The diffraction peak indexed to the (001) plane of 1T-MoS 2 shifts from 14.07°( 0.629 nm) to 13.59°(0.651 nm) when Zn SAs are present, which corresponds to an ∼3.4% (0.022 nm) increase in 1T-MoS 2 's interlayer spacing. Likewise, the peak indexed to the (101̅ ) plane of Mo 2 S 3 shifts from 16.28°(0.544 nm) to 15.72°( 0.563 nm) in Zn SAs/1T-MoS 2 and also corresponds to an ∼3.4% (0.019 nm) expansion. DFT calculations reported in the literature have predicted a decreased diffusion energy barrier for SA migration between 2D layers as the interlayer spacing increases. 46,47 Here, the interlayer spacing of 1T-MoS 2 expands to accommodate the occupation of the interlayer lattice sites by Zn SAs.
Comparing the distance between lattice fringes in 1T-MoS 2 to those found in Zn SAs/1T-MoS 2 ( Figure 2d) generates similar results. Images produced by high resolution TEM (HRTEM) with the electron beam aligned parallel to the 1T-MoS 2 basal plane show an average distance of 0.632 ± 0.06 nm for lattice fringes found in 1T-MoS 2 ( Figure 2c). Lattice fringes present in Zn SAs/1T-MoS 2 yield an average distance of 0.661 ± 0.09 nm (Figure 2d), which is roughly 0.029 nm larger than the 1T-MoS 2 lattice fringes observed when Zn SAs are absent. This result corresponds to a 4.6% average increase in 1T-MoS 2 's interlayer spacing. Altogether, the XRD and HRTEM results both suggest that intercalating Zn SAs expands 1T-MoS 2 's interlayer spacing.
Structural Characterization of Zn SAs/1T-MoS 2 . The Zn SAs/1T-MoS 2 was analyzed by STEM and energydispersive X-ray (EDX) spectroscopy to atomically visualize the layer geometry and confirm the elemental components To understand how the confinement of Zn SAs impacts the electronic structure of 1T-MoS 2 , X-ray photoelectron spectroscopy (XPS) was employed to compare the elemental and chemical compositions of Zn SAs/1T-MoS 2 and 1T-MoS 2 ( Figure S5, Table S1). Peaks identified in the Zn 2p spectrum of Zn SAs/1T-MoS 2 correspond to the presence of Zn 2+ and confirm that the SAs retain their Zn 2+ oxidation state after intercalation ( Figure S5g). 48 Deconvolution of peaks in the Mo 3d, S 2s ( Figures S5a,b), and S 2p (Figures S5c,d) spectra produces two sets of doublet peaks, in which the set of peaks at lower binding energies (shown in purple) are assigned to Mo 4+ and S 2− oxidation states in 1T-MoS 2 . 48 The sets of doublet peaks at higher binding energies (shown in green) correspond to unsaturated Mo 5/6+ and S 2− oxidation states and indicate the presence of a nonstoichiometric MoS x species along the surface of each sample. 48 Consequently, while the majority of both samples consists of stoichiometric 1T-MoS 2 , a portion of MoS x exists near the catalyst surface. 49 Compared to 1T-MoS 2 , Zn SAs/1T-MoS 2 displays downshifts in binding energies of up to 0.19 eV. Downshifts of up to 0.10 eV fall within the step size (0.10 eV) employed during XPS analysis and as such are considered negligible. However, peak shifts exceeding the step size employed are observed for the S 2p doublet assigned to MoS x (Figures S5c,d). This shift to lower binding energies indicates that the Zn SAs donate electrons to the S atoms of MoS x . 50 This electron donation likely occurs to facilitate the stabilization of Zn SAs within the substrate's interlayer spacing. Further evidence of the lack of structural changes to 1T-MoS 2 upon intercalation of Zn SAs is provided by the FTIR spectra, which also demonstrates negligible chemical and electronic structure changes ( Figure S6).
Next, Raman spectroscopy was employed to explore key structural details, such as lattice strain and vacancy defects. The Raman spectra for 1T-MoS 2 and Zn SAs/1T-MoS 2 are displayed in Figure S7 with experimental peak positions listed in Table S2. The existence of the 1T-phase in both samples is confirmed by the presence of transverse acoustic (TA) and longitudinal acoustic (LA) phonon modes at the M point of the first Brillouin zone and J 1 , J 2 , E 1g , and J 3 phonon modes. The negligible difference in the 1T-MoS 2 and Zn SAs/1T-MoS 2 peak positions provides further evidence that the lattice structure of 1T-MoS 2 remains intact in the presence of Zn SAs. 51 To compare differences in the magnetic properties and the amount of S vacancies, Zn SAs/1T-MoS 2 and 1T-MoS 2 were both evaluated by electron paramagnetic resonance (EPR) spectroscopy. Additionally, 2H-MoS 2 was evaluated and treated as a control during EPR analysis. The EPR spectrum of 2H-MoS 2 displays a narrow line shape and isotropic g-value (g = 2.005) that are attributed to dangling Mo−S bonds generated by S vacancies ( Figure S8a). 25,52 In contrast, 1T-MoS 2 ( Figure S8b) and Zn SAs/1T-MoS 2 ( Figure S8c) both produce complex EPR spectra in which more than one paramagnetic center is present. Since 1T-MoS 2 and Zn SAs/ 1T-MoS 2 yield similar g-values, the following assignments of paramagnetic centers apply to both samples. Direct correlation of the specific g-values each paramagnetic center is assigned to is provided in Table S3. The first paramagnetic center identified corresponds to paramagnetic S atoms in short chains (g = 2.042) and electron hole centers localized on S atoms (g ≈ 2.026). 53 The second paramagnetic center, which generates g-values between 1.932 to 1.959 and 2.017 and 2.019, is assigned to Mo 5+ species coordinated to S atoms. 54−56 The existence of Mo 5+ species corresponds to the presence of Mo species at higher oxidation states (Mo 5/6+ ) in the XPS spectra ( Figure S5) and is a result of local structural defects in 1T-MoS 2 that give rise to under-coordinated Mo atoms within the substrate. 57 The signals observed for g-values between 1.993 to 2.005 are ascribed to S−Mo 5+ defects and dangling Mo−S bonds generated by S vacancies, respectively. 52,57 The negligible difference in signal intensity of 1T-MoS 2 from that of Zn SAs/1T-MoS 2 indicates that the concentration of S vacancies is similar in both samples. 25,52,58 Therefore, the influence of S vacancies on the catalytic performance of Zn SAs/1T-MoS 2 is excluded. Coordination Environment and Valence States of Zn SAs/1T-MoS 2 . To confirm the chemical states and atomic dispersion of the Zn SAs, the electronic and coordination structures of Zn SAs/1T-MoS 2 were studied by X-ray absorption spectroscopy (XAS) at both the Zn K-edge ( Figure  3) and the Mo K-edge ( Figure S9). The X-ray absorption near edge structure (XANES) spectra ( Figure 3a) show that the white line intensity and absorption edge of Zn SAs/1T-MoS 2 are closer to ZnO than those of Zn foil, indicating that the SAs exist in the Zn 2+ oxidation state. This is also reflected by the first derivative of the XANES spectra (Figure 3b). The k 3weighted extended X-ray absorption fine structure (EXAFS) spectra ( Figure 3c) and Fourier-transformed EXAFS spectra in R-space (Figure 3d) correlate well with their best fitting lines modeled by DFT (Figure 5b), respectively, and suggest that Zn exists as SAs tetrahedrally coordinated to 4 S atoms. The coordination environment adopted by Zn SAs under confinement was first modeled as 1T-MoS 2 layers confining a Zn SA that is octahedrally coordinated to the S basal planes. After structural optimization, the SA's coordination geometry reorganized into a tetrahedral coordination with the S basal planes. Specifically, the Zn SAs formed asymmetric Zn−S1 and Zn−S3 coordination structures with the upper and lower S layers with bond lengths equal to 2.31 Å (Table S4). Typical peaks correspond to Zn−Zn bond formation (>2.50 Å) were not observed (Figure 3d), which indicates that the SAs remain atomically dispersed when under confinement. 59,60 This

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Article tetrahedral coordination enables the Zn SAs to stabilize the expanded interlayer spacing of 1T-MoS 2 , which in turn, retains greater exposure of basal plane S active sites for enhanced HER performance. Figure S9a shows the XANES spectra of Mo foil, 1T-MoS 2 , and Zn SAs/1T-MoS 2 at the Mo K-edge corresponding to the 1s−5p transition. The edge energy of Zn SAs/1T-MoS 2 is visibly higher than Mo foil and very close to 1T-MoS 2 , suggesting that the Mo atoms in Zn SAs/1T-MoS 2 have an oxidation state similar to that of 1T-MoS 2 . This conclusion is further verified by the first derivative spectra of XANES ( Figure S9b), where the valency of the Mo atoms in Zn SAs/ 1T-MoS 2 is nearly the same as that of 1T-MoS 2 . The Mo Kedge EXAFS spectra in K-space ( Figure S9c) and Fouriertransformed EXAFS spectra in R-space ( Figure S9d) coincide well with the fitting line (also based on the DFT model in Figure 5b), indicating that the Mo atoms in Zn SAs/1T-MoS 2 are octahedrally coordinated to 6 S atoms and an increase in S vacancies is not observed. The fitting results of the Fouriertransformed EXAFS spectra in R-space (Table S5) show that while half of the Mo−S bonds are at lengths that are expected for 1T-MoS 2 (2.41 Å), the other half exhibit longer bond lengths equal to 2.62 Å (Table S5). This increase in the bond length may be ascribed to the slight distortion of octahedrally coordinated Mo−S centers induced by the intercalation of Zn SAs.
Altogether, these results confirm a few key findings. First, the Zn SAs adsorb between layers of 1T-MoS 2 and are stabilized by Zn−S bonding interactions. Second, intercalating Zn SAs between layers of 1T-MoS 2 expands the interlayer spacing by ∼3.4%. Third, the incorporation of Zn SAs does not influence the electronic properties or concentration of S vacancies as compared to that of pristine 1T-MoS 2 . Instead, the Zn SAs retain their 2+ oxidation state and d 10 electronic configuration, which inhibits their ability to perform as active sites for the HER. Lastly, the Zn SAs tetrahedrally coordinate to basal S atoms and induce slight distortion of up to half of the Mo−S bonds in 1T-MoS 2 . Based on these findings, the impact of Zn confinement on HER catalysis in 1T-MoS 2 is believed to solely be caused by confinement effects.
HER Activity of Zn SAs/1T-MoS 2 . Changes in HER performance with intercalation of Zn SAs were monitored electrochemically under acidic conditions (N 2 -saturated 0.5 M H 2 SO 4 ) within a three-electrode configuration (detailed in the Methods). Linear sweep voltammograms (LSVs) were collected to measure the amount of overpotential (measured at −10 mA/cm 2 ) 1T-MoS 2 requires to drive HER and elucidate how the overpotential changes when Zn SAs are confined near basal plane active sites in 1T-MoS 2 . Comparison of overpotentials yielded by 1T-MoS 2 intercalated with 2.5, 8.5, and 16.5 mg of Zn SAs shows that the overpotential peaks at 177 mV vs RHE with 8.5 mg of Zn SAs intercalated into 1T-MoS 2 ( Figure S10a, Table S6). The same trend is observed when the electrochemical impedance spectra (EIS) of the samples are analyzed to evaluate charge transport limitations ( Figure S10b). The lowest charge transfer resistance (18.41 Ω) is observed when 8.5 mg of Zn SAs is intercalated into 1T-MoS 2 (Table S7). These results indicate that intercalating 8.5 mg of Zn SAs enhances 1T-MoS 2 's catalytic performance, while quantities above or below this amount exhibit worse HER activity and slow charge transport efficiency. When 8.5 mg of Zn SAs is intercalated, 1T-MoS 2 's overpotential was reduced by 88 mV (Figure 4a, Table S6), indicating that less energy is required to drive HER when Zn SAs are spatially confined within 1T-MoS 2 's interlayer spacing (Figure 4a).
Tafel slopes were extracted from the onset potential region of the LSVs to identify how the presence of Zn SAs changes the rate at which HER proceeds on 1T-MoS 2 in acidic media (Figure 4b). In the first step of the HER mechanism, known as the Volmer step, a proton is reduced at an active site and adsorbed on the catalyst's surface. In the second step, H 2 is released through one of two pathways: either by a second proton/electron transfer, known as the Heyrovsky step, or the recombination of two adsorbed protons, known as the Tafel step. 61 Since catalysts that exhibit the best HER performance yield the lowest overpotentials, these catalysts will produce the lowest Tafel slopes. 8 Comparison of Tafel slopes produced by 1T-MoS 2 (107.2 mV/dec) and Zn SAs/1T-MoS 2 (84.9 mV/ dec) shows a decrease of 22.3 mV/dec when Zn SAs are intercalated between 1T-MoS 2 layers (Figure 4b). Spatial confinement has been predicted to increase the electrochemical potential of confined reacting species, which in turn, enhances charge transfer between the catalyst surface and adsorbed protons. As a result, less applied voltage is needed to produce the same current density. 20 Here, the lowered kinetic barrier through which the HER mechanism proceeds is evidenced by the decrease in the Tafel slope observed when Zn SAs are under confinement. Comparison of these results to similar cases of SACs reported in literature reveals a similar trend in overpotential and Tafel slopes for 2H and 1T MoS 2 supports intercalated with first-row transition metals that exhibit similar electronic configurations to Zn 2+ (Table S8).
Confinement-induced changes in the charge transfer properties were evaluated by EIS (Figure 4c). A reduction in charge transfer resistance when Zn SAs are spatially confined within 1T-MoS 2 is evidenced by the observably smaller radius of the semicircle produced by Zn SAs/1T-MoS 2 compared to the radius of the semicircle produced by 1T-MoS 2 . Numerically, Zn confinement reduces charge transfer resistance in 1T-MoS 2 from 127.5 to 18.41 Ω. This decrease in charge transfer resistance implies that the rate of charge transfer from the active sites to the adsorbed protons is increased with interlayer confinement of Zn SAs.
In catalysis, the rate of an electrocatalytic reaction is directly proportional to the active surface area. Therefore, cyclic voltammetry (CV, Figure S11) was employed to derive the double-layer capacitance and calculate the electrochemically active surface area (ECSA) of 1T-MoS 2 and Zn SAs/1T-MoS 2 (Figure 4d). Compared to the ECSA 1T-MoS 2 yields (7 mF/ cm 2 ), Zn SAs/1T-MoS 2 yields an ECSA equal to 29 mF/cm 2 , roughly four times greater than 1T-MoS 2 's ECSA. As a measure of the catalyst's surface area that is accessible to the electrolyte, this improvement implies that there is roughly four times more area available to facilitate charge transfer when Zn SAs are confined near basal plane active sites. This result corresponds well with the EIS results and may be correlated to the increase in fringe spacing induced by the Zn SAs in 1T-MoS 2 that was observed in the HRTEM and XRD results (Figure 2).
To test the stability of Zn SAs/1T-MoS 2 during catalysis, LSVs were collected both before and after the catalyst was subjected to 3,000 CV scans (Figure 4e). After 3,000 cycles, the overpotential increased by only 14 mV at 10 mA/cm 2 . The excellent stability was further evidenced by continuous electrolysis at −0.2 V vs RHE which showed nearly unchanged current generation after 24 h (∼20 mA cm −2 , Figure S12). XPS ACS Nano www.acsnano.org Article spectra of Zn SAs/1T-MoS 2 on carbon fiber paper (CFP) were collected before and after collecting 3,000 CV scans to evaluate the catalyst's structural stability ( Figure S13, Table S9)  4 2− present on the sample grows ( Figures S13f,h). Specifically, the sample initially starts with 4.55 at% of SO 4 2− . After CV is completed, 20.56 at% of SO 4 2− is observed on the sample's surface. Furthermore, the Zn 2p peaks reappear after the sample is sputtered for 30 s ( Figure S14). Based on these results, the following observations were made. O 2 has been reported in the literature to form Mo-oxide species by bonding to unsaturated Mo atoms at vacancy sites on defect-rich MoS 2 surfaces via chemical adsorption. 62,63 Furthermore, O 2 has been reported to not alter the electronic properties of MoS 2 . 63 Therefore, the complete loss of MoO 3 after electrolysis indicates that this species is localized along the sample surface and is likely caused by surface oxidation during working electrode preparation. Meanwhile, interactions between the electrolyte and catalyst surface during HER drive the growth of SO 4 2− on the catalyst surface. The significant increase of SO 4 2− on the catalyst surface likely inhibits the ability to see the Zn 2p peaks after electrolysis, considering that the concentration of intercalated Zn SAs is very low (1.0% as determined by ICP-OES, Table S10). Since SO 4 2− is a soluble ion, once the sample is submerged into the electrolyte, SO 4 2− may redissolve. The reappearance of Zn 2p peaks after sputtering the sample for 30 s indicates that the Zn SAs intercalated between layers beneath the catalyst surface remain intact. The lack of significant decline (14 mV shift) in HER performance after 3,000 CVs observed further confirms that the integrity of the catalyst structure is well maintained (Figure 4e).
As previously mentioned, an average mass percent of 1.0% Zn SAs in Zn SAs/1T-MoS 2 as quantified by ICP-OES (Table  S10) was employed to determine turnover frequencies (TOF) and evaluate the catalyst's efficiency toward HER (Figure 4f). Similar to the polarization curves, the TOF values increase with higher potential. This result aligns well with reports from literature as well. 64 In our previous work, 1T-MoS 2 substituted with Ni SAs yielded a TOF of 0.7 s −1 at 130 mV overpotential. 37 Here, the intercalation of Zn SAs between layers of 1T-MoS 2 yields a TOF equal to 1.40 s −1 at 177 mV overpotential when the current density is 10 mA/cm 2 . Overall, the effects of Zn confinement within 1T-MoS 2 layers include decreased overpotentials, lowered kinetic barriers , faster charge transfer rates, and increased active surface area, all while retaining excellent stability.
Correlation of Confinement Effects to HER activity. First-principles DFT calculations were performed to elucidate how confined Zn SAs influence 1T-MoS 2 's catalytic performance. Initially, two positions occupied by a Zn SA located above the basal plane of a single layer of 1T-MoS 2 were considered ( Figure S15). In the first structure, the Zn SA was positioned above a Mo atom (single layer Zn SAs/1T-MoS 2 , Model I). In the second structure, the Zn SA is positioned above an S atom (single layer Zn SAs/1T-MoS 2 , Model II). Compared to the normal length of Zn−S bonds (∼2.36 Å), both coordination structures yield elongated Zn−S bond lengths that are greater than 2.95 Å. 65 Therefore, the formation of Zn SAs/1T-MoS 2 is predicted to be unfavorable for Models I and II with only a single layer of 1T-MoS 2 . Instead, two or more layers of 1T-MoS 2 are required to stabilize intercalated Zn SAs and properly investigate the catalytic effects of spatially confining Zn SAs within the microenvironment of the 1T-MoS 2 active sites.
In an effort to understand why the Zn SAs prefer to adsorb to the basal plane on top of Mo atomic positions instead of substituting within the 1T-MoS 2 lattice, local active configurations demonstrating the possible atomic positions of Zn SAs on 1T-MoS 2 were constructed and their resulting formation energies for each configuration were compared. In total, there are four possible anchoring sites for Zn SAs on 1T-MoS 2 ( Figure S16). The reported chemical potentials used to calculate the formation energies discussed herein for S, Mo, and Zn were taken from an S8 molecule, Mo metal, and Zn metal, respectively. In the first model ( Figure S16a), a Zn SA was tetrahedrally coordinated to 1T-MoS 2 with one bond formed with the top layer's basal plane and three bonds formed with the bottom layer's basal plane, respectively. In this configuration, the Zn atom is adsorbed on top of an Mo atom, and the formation energy is −6.06 eV. In the second model ( Figure S16b), a Zn SA was tetrahedrally coordinated to form three bonds with the upper basal plane and one bond with the lower basal plane. However, the instability of this configuration caused the model to rearrange itself into a tetrahedral coordination that forms two bonds with the upper basal plane and two bonds with the lower basal plane. In this configuration, the Zn atom adsorbs on top of an S atom and the formation energy is −5.77 eV. In the third configuration ( Figure S16c), an Mo atom is substituted by a Zn atom, which generates a formation energy equal to −2.14 eV. In the fourth configuration ( Figure S16d), a Zn atom substitutes an S atom in the lower basal plane, which generates a formation energy equal to −4.16 eV. Comparison between the different formation energies of the four configurations shows that the first configuration with the most negative formation energy ( Figure S16a, −6.06 eV) is the most likely configuration to occur due to being the most thermodynamically stable configuration. This result corresponds with the experimental STEM results (Figure 1).
To understand how confining Zn SAs within the microenvironment of 1T-MoS 2 active sites changes the proton adsorption/desorption kinetics of 1T-MoS 2 , ΔG H* values for a trilayer of 1T-MoS 2 and a trilayer 1T-MoS 2 intercalated with a tetrahedrally coordinated Zn SA were calculated for HER at a potential of 0 V vs RHE and pH = 0 ( Figure 5). Predicted models of the top view and optimal adsorption modes of H* for both cases are demonstrated in Figures 5a, 5b. The extremely negative ΔG H* value (−5.40 eV) yielded by trilayer 1T-MoS 2 indicates that the adsorption of protons to the S active sites will be strong. Consequently, the desorption of adsorbed protons upon producing H 2 will be challenging, and sluggish reaction kinetics are expected for active sites located along the basal plane. In contrast, Zn SAs/1T-MoS 2 yields a much more thermoneutral ΔG H* (0.00294 eV), indicating that HER adsorption and desorption kinetics will be much more facile when Zn SAs are confined near the basal plane active sites in 1T-MoS 2 . Altogether, these calculations correspond well with the experimentally observed improvement in the HER performance.
Mulliken charge analysis was employed to predict the changes in the electron density of S active sites along the basal plane when Zn SAs are confined within their microenvironments ( Figure S17). Here, 3D isosurface diagrams of the differential charge are displayed for Zn SAs/1T-MoS 2 . Charge clouds shown in blue represent energetically negative regions where the atom has lost electrons. Charge clouds shown in red exhibit positive charge and represent atoms that have gained electrons. The tetrahedral coordination of Zn SAs to basal S atoms yields a thermoneutral free energy value, making this coordination structure ideal for fast HER kinetics. In this coordination structure, the Zn atom donates electrons to nearby basal plane S atoms on 1T-MoS 2 , resulting in the formation of strong ZnS bonds. This electron transfer changes the adsorption behavior of H atoms attached to neighboring S active sites. Based on these predictions, one may deduce that confining Zn SAs enhances the catalytic activity of 1T-MoS 2 by increasing the electron density surrounding the S active sites around SAs. As a result, the catalytic activity of 1T-MoS 2 is enhanced accordingly.
To reveal the influence of Zn intercalation on the band structure of 1T-MoS 2 , the PDOS spectra of the Zn 4s (black), Zn 3d (blue), S 3s (green), and S 3p (purple) orbitals were evaluated ( Figure S18). Results show a strong electronic overlap between the Zn 4s and S 3p orbitals. Together with the Mulliken charge population analysis of the atomic orbitals ( Figure S17), one may deduce that the electrons transfer from the Zn 4s orbital to the S 3p orbital during Zn−S bond formation, which in turn causes the Zn atom to lose electrons and exhibit a positive valence state. Consequently, the S atoms coordinated to the adsorbed Zn atom gain electrons and exhibit a more negative valence state than they would in bare 1T-MoS 2 . These results match well with the experimental XPS results reported herein ( Figure S5). In regard to the effects of electronic accumulation at S sites on H* adsorption, near the Fermi level the density of states mainly comes from S atoms, while the contribution from Zn is close to 0 eV. Therefore, the Zn atoms exhibit weak adsorption capacity for H* and are thus inactive, which is consistent with the H* adsorption configuration. In contrast, the S atoms have available p orbitals that can form bonds with H*. The observed electron transfer from Zn to neighboring S atoms (Figures S17 and S18) leads to an increase in electron filling and decrease in empty orbitals near the Fermi level, which weakens the electron accepting ability of S sites and H* adsorption.
To further elucidate the influence of Zn intercalation on the electronic structure and resulting HER activity of 1T-MoS 2 , the PDOS spectra of the s and p orbitals in 1T-MoS 2 and Zn SAs/1T-MoS 2 were compared ( Figure S19). As elucidated from Figure S18, electron transfer from Zn to neighboring S atoms results in a higher extent of electron filling of neighboring S atoms. As shown by Figure S19a, this leads to a negative shift in the p band center of S atoms in Zn SAs/1T-MoS 2 from −3.78 eV (red dashed line) to −4.0 eV (green dashed line) and a decrease in the intensity of state density compared to 1T-MoS 2 near the Fermi level (shown in the black dashed box). Since the electronegativity of the S atom is stronger than a proton's electronegativity, the protons will donate electrons to active S sites in the adsorption process. However, the increase in electronic occupation of the S 3p orbitals and decrease in empty orbitals near the Fermi level caused by Zn intercalation will cause a reduction in the electron accepting ability of the S atoms. Although the adsorption of protons is not strictly linear with the atomic electron density of S, it is negatively correlated. Thus, the number of electrons transferred to 1T-MoS 2 induced by proton adsorption is reduced after Zn intercalation. Proton adsorption capacity reduces from −5.40 to 0.5819 eV after Zn intercalation, which in turn sharply increases the catalyst's HER activity.
Since Zn intercalation influences the electronic structure of the catalyst's entire surface, we also calculated the H* adsorption behavior of some typical sites ( Figure S20). The results suggest that Zn intercalation causes the HER activity of the catalyst's entire surface to improve, as shown by the resulting ΔG H* values being closer to 0 eV for all of the H* adsorption sites considered. Meanwhile, compared with the neighboring S atoms in the first coordination sphere, the adsorption capacity of neighboring S atoms in the second coordination sphere is also weakened and yields better HER activity than the S atoms in the first coordination sphere (0.00294 eV).
To explore the influence of interlayer spacing expansion on the HER activity of 1T-MoS 2 , we manually set the lattice parameters of the supercell to increase by 3.4% along the Z axis ACS Nano www.acsnano.org Article before structural optimization ( Figure S21a). By fixing the lattice parameters, only the structure of the ion step can be changed during structural optimization ( Figure S21b). Calculation results show a ΔG H* of −4.8081 eV, far worse than that of Zn SAs/1T-MoS 2 (0.00294 eV). In this regard, the electronic regulation of neighboring S atoms is believed to be the root cause of the observed improvement in HER activity upon intercalation of Zn SAs between layers of 1T-MoS 2 . By comparison, the lattice spacing expansion is only a secondary factor.

CONCLUSIONS
In this work, the catalytic effects of confining Zn SAs within the interlayer spacing of 1T-MoS 2 were investigated, and several key findings were revealed. DFT and PDOS calculations predict Zn SAs/1T-MoS 2 to yield a much more thermoneutral ΔG H* (0.00294 eV) compared to that of 1T-MoS 2 (−5.40 eV), suggesting that HER adsorption and desorption kinetics will be much more facile when Zn SAs are under confinement. This work reveals that intercalating transition metal ions such as Zn 2+ into catalytic layered materials such as 1T-MoS 2 enables the electronic states of confined microenvironments to be controlled according to the guest metal's coordination geometry and electronic states. In turn, the 2D support's catalytic activity may be enhanced accordingly. Although this work investigates the confinement of Zn SAs within the microenvironments of 1T-MoS 2 's basal plane active sites and its influence on HER performance, the basis of this work may easily be adopted to other types of catalytic reactions and layered materials. Since confinement effects have been recognized for their importance in heterogeneous, homogeneous, and enzymatic catalysis, the knowledge gained from this work may be appropriately applied to all fields of catalysis. Synthesis of 1T-MoS 2 Nanosheets. First, 50 mg of (NH 4 ) 6 Mo 7 O 24 ·6H 2 O, 80 mg of CH 3 CSNH 2 , and 10 mL of deionized (DI) water were combined in a 25 mL autoclave and sonicated until they were fully dissolved. Once dissolved, the autoclave was sealed within a hydrothermal reactor and heated at 180°C for 24 h. Once the reactor cooled to room temperature, the solution of crude product was transferred to 15 mL conical centrifuge tubes. The crude product was washed and centrifuged three times: once with DI water, once with ethyl alcohol, and once with acetone. The solutions were centrifuged for 10 min at ∼4,000 rpm in between each washing step and the supernatant layer was removed after each centrifugation step. After washing, the purified product was dried in a vacuum oven at ∼80°C and ∼25 mmHg for 24 h. The dried and purified product was finely ground with a mortar and pestle and stored under ambient conditions. Synthesis of Zn SAs/1T-MoS 2 . First, 50 mg of 1T-MoS 2 , 15 mL of DI water, and 35 mL of ethanol were combined and continuously stirred at room temperature during the entire synthesis. Next, 2.5, 8.5, and 16.5 mg of ZnCl 2 was dissolved in 6 mL of DI water to prepare Zn SAs (2.5 mg)/1T-MoS 2 , Zn SAs (8.5 mg)/1T-MoS 2 , and Zn SAs (16.5 mg)/1T-MoS 2 , respectively. A syringe pump apparatus was employed to inject the Zn solution into the 1T-MoS 2 solution. To do this, the entire solution of ZnCl 2 was injected into the 1T-MoS 2 solution at a flow rate of 10 μL/min flow rate. The solution stirred continuously at room temperature for 24 h after the injection was complete. Afterward, the solution was transferred to 15 mL Conical Centrifuge Tubes, centrifuged for 10 min at ∼4,000 rpm, washed with DI water, and centrifuged again for the same amount of time. The supernatant layer was removed after each centrifugation step. The purified product was dried in a vacuum oven at ∼80°C and ∼25 inHg for 24 h, then finely ground with a mortar and pestle and stored under ambient conditions.

METHODS
Characterization. XRD patterns were measured using a Panalytical X'Pert multipurpose XRD with Cu−Kα radiation (λ = 1.5418 Å) at 45 kV and 40 mA, with step size = 0.02 and scan step time = 17.75 s. The measurement range was from 10°to 80°in terms of 2θ. XPS measurements were collected with a PHI 5600 XPS system equipped with a monochromatic Al Kα X-ray source and Omni Focus III lens operating at 250 W, 14 kV, 600 μm 2 spot size, and a maximum base pressure of 5 × 10 −9 Torr. A 90°angle was maintained between the X-ray source and analyzer. Survey spectra were collected with 117.4 eV pass energy, 1.0 eV/step, and 50 μs dwell time. Multiplexes were collected using 11.75 eV pass energy, 0.050 eV/step, and 50 μs dwell time. The instrument was calibrated to Au 4f 7/2 = 84.00 eV and Cu 2p 3/2 = 932.67 eV immediately prior to collecting the data. All spectra were calibrated to C 1s = 284.8 eV. 66 Multiplexes were fitted by using IgorPro XPS Tools Software. A Shirley background and 80% Lorentzian−Gaussian were employed for all peak analyses. FTIR spectra were collected using a PERKIN ELMER CE-440. Raman spectra were recorded using a Thermo Scientific DXR Raman Spectrometer employing an Ar-ion laser operating at 532 nm, a 50 μm pinhole, and 3.0 mW laser power. EPR characterization was carried out on a Bruker EMX spectrometer (X-band) operating at a frequency of ∼9.46 GHz. Field frequency modulation, modulation amplitude, and microwave power were set to 100 kHz, 0.4 mT, and 2.0 mW, respectively, in every case to avoid saturation effects. All EPR measurements were recorded at room temperature.
Aberration-corrected scanning transmission electron microscopy (AC-STEM) was performed using the JEOL Grand ARM equipped with two spherical aberration correctors at 300 kV. High-angle angular dark-field (HAADF) STEM images were acquired by a convergence semiangle of 22 mrad and inner and outer collection angles of 83 and 165 mrad, respectively. Energy dispersive X-ray spectroscopy (EDX) was conducted by using JEOL dual EDX detectors and a specific high count analytical TEM holder. Sample compositions were analyzed by a PerkinElmer Optima 3000 DV ICP-OES. Commercially available Copper standard solutions (1000 mg L −1 in nitric acid, Sigma-Aldrich) were used for calibration. The standards were diluted to filters. Mo K-edge XAS data was measured within the range 19.778− 20.887 keV in fluorescence mode with a step size of 0.25 eV at the near edge. The Zn K-edge XAS was run within the 9.46−10.50 keV range in fluorescence mode with a step size of 0.25 eV at the near edge. All samples were prepared by placing a small amount of homogenized powder mixed with boron nitride (via agate mortar and pestle) on 3 M Kapton Polyimide tape, which was purchased from 3M (https://www.3m.com/). Electrochemical Measurements. All electrochemical measurements were conducted in a N 2 -saturated 0.5 M H 2 SO 4 electrolytic solution within a three-electrode configuration. A CHI 660E electrochemical workstation was used for all electrochemical measurements. Graphite was employed as the counter electrode and Ag/AgCl (3 M NaCl, BASI) as the reference electrode. Electrodes were prepared by drop casting 200 μL of catalyst ink (50 μL each time, repeated 4 times) onto a 1 × 2 cm 2 piece of carbon fiber paper (CFP). The loading of the catalysts on CFP is 1 mg/cm 2 . All potentials reported were calibrated with respect to the Ag/AgCl reference electrode in acidic media (0.5 M H 2 SO 4 ) using eq 1: LSVs were conducted under ambient conditions from 0 to −0.8 V with a 5 mV/s scan rate, 1 mV step size, and 0.001 A/V sensitivity. Onset potentials used to determine the Tafel slopes were extracted from the LSVs and defined as the potential at which the current began to increase (0.05 mA/cm 2 ). Overpotentials were measured at −10 mA/cm 2 . EIS measurements were performed at −0.4 V with a 0.005 V variation in the frequency range of 1−10 5 Hz and 12 steps per decade. The electrochemical active surface area (ECSA) was derived from cyclic voltammograms (CV) measured with varying scanning rates of 20, 40, 60, 80, and 100 mV s −1 . The stability of Zn SAs/1T-MoS 2 was evaluated by running CV for 3,000 cycles from 0 to −0.8 V, followed by comparison of the initial and final LSV curves. The turnover frequency (TOF) was reported as the TOF corresponding to the overpotential reported (177 mV vs RHE). Computational Methods. Density functional theory (DFT) calculations were performed by using CASTEP coding. The electronic exchange-correlation potential was conducted using the Perdew− Burke−Ernzerhof (PBE) functional of the generalized gradient approximation (GGA) and the ultrasoft pseudopotentials were used. The kinetic energy cutoff was set to 400 eV for the plane-wave basis set. Brillouin zone integration was sampled with the 3 × 3 × 1 MonkhorstPack mesh K-point for bulk and surface calculations, respectively. The DFT dispersion correction (DFT-D) method was used to correct for the van der Waals interactions. A three-layer repeating unit supercell with the formula Mo 27 S 54 and Mo 27 S 54 Zn were constructed for bulk calculations. Monolayered 1T-MoS 2 (Mo 9 S 18 ), Zn SAs/1T-MoS 2 (Mo 9 S 18 Zn), with a vacuum region of 15 Å along the Z axis, was constructed based on the HAADF-STEM imaging. The convergence tolerances were set to 1 × 10 −5 eV per atom for energy, 1 × 10−3 Å for maximum displacement, and 0.03 eV Å −1 for maximum force. The thermodynamic energies and Gibbs free energies ΔG H* were calculated using eq 2:
HAADF-STEM images, FTIR spectra, EDX mapping, ICP-OES analysis, Raman spectra and assignment, XPS spectra and assignment, EPR spectra and g-value assignments, XAS characterization and fitting parameters, electrochemical data and fitting, DFT models, and Mulliken charge analysis (PDF)